The present invention relates generally to procedures for cleaning contaminated groundwater, and more particularly to in-situ procedures for removing volatile organic compounds (VOCs) from groundwater.
During the last decade, over 1200 hazardous waste sites in the U.S. have been placed on the EPA National Priorities List for remedial investigation and cleanup. However, current remediation methods are often very expensive, and alternative concepts and techniques are needed. The present invention is particularly concerned with VOCs which pose a significant threat to groundwater supplies and are commonly detected in groundwater.
The most common class of VOC pollutants are petroleum products, such as gasoline and jet fuels. The U.S. EPA estimates that there are more than 2 million underground tanks in the United States and that 20 percent of them leak and contaminate groundwater. Such tanks leak benzene, toluene and their derivatives which become dissolved in groundwater. Another important group of VOC pollutants is chlorinated hydrocarbons, notably TCE (trichloroethylene, CHCl.dbd.CCl.sub.2) and its degradation products.
One challenge for hydrologists and environmental engineers is to develop new in-situ remediation methods for removing the dissolved organic contaminants in a simple, cheap and efficient manner. At many contaminated sites, it is common for the majority of the organic pollutants to exist as separate liquid phases. A portion may dissolve into groundwater or may evaporate into the gas phase in the unsaturated zone. Once in the groundwater, the dissolved organic contaminants are transported as plumes. During aquifer remediation, the main body of organic liquid is usually removed from groundwater by skimming or pumping with subsequent above ground treatment. A portion of the liquid phase that is retained by capillary forces may continue to slowly dissolve. Remedial action may also include forced vacuum extraction through the unsaturated zone to remove the gas phase of the toxic substances. Unfortunately, the dissolved portion is not treated in-situ, because remediation technologies are limited to `pump-and-treat` (i.e., above ground) methods.
The newly-developed method of biorestoration may provide an alternative for some specific cases. This method is aimed at enhancing biodegradation of organic compounds through the introduction or stimulation of natural microorganisms along with injection of nutrients and oxygen. Lately, methanogenic microorganisms have been discovered in natural systems that are able to co-metabolize TCE under reducing conditions after controlled stimulation. Although in the early phase of technology development, these methods may become practical and effective. However, these methods are limited to very specific conditions; for example, in-situ bioremediation of TCE-containing water is apparently limited to fluids containing less than about 100 ppm of TCE, as higher concentrations seem to be toxic. Alternatively, an in-situ remediation technique using an under-pressure vaporizer floating device was introduced.
Apparently, an in-situ aquifer remediation method that employs air-lift pumping as a means of producing gas bubbles to remove VOCs from groundwater is not mentioned in the literature. Related studies have inspected the effects of air bubbles on various hydrologic, geologic, and engineering processes. The general behavior of air bubbles in groundwater is mentioned in the hydrological literature in relation to its effect on decreasing hydraulic conductivity, its effect on soil moisture hysteresis and its effect on water table fluctuations. It has been suggested that air bubbles might serve as carriers of suspended particles such as clay minerals in porous media, due to their special interface properties. In the petroleum engineering literature, the behavior of gas bubbles is mentioned by researchers regarding their effect on oil reservoirs. Transport by gas bubbles in the free liquid phase has received attention in fields as diverse as oceanography, where bubbles are mentioned as important carriers of organic matter to the sea surface, and in flotation techniques for the processing of ores.
Air-liquid mass exchange has been applied in two different processes. First is "gas stripping" of industrial wastewater using large towers above the ground; and second, the purge and trap laboratory technique for analysis of concentrations of trace volatile chemicals. This water-gas phase mass transfer is very efficient. In the case of gas stripping it is possible to reduce concentrations in the aqueous phase to the water quality standards which are frequently at the detection limit. It is interesting to note that today gas stripping is used as a standard method for removing volatile chemicals from pumped groundwater in contaminated sites before its supply for domestic usage. The purge-and-trap method in the laboratory is also an effective removal method for many compounds.
The present invention involves a new concept for in-situ removal of dissolved VOCs from the saturated zone. It avoids standard `pump-and-treat` methods. It is a combined gas-lift pumping technique and in-situ vapor stripping method. The idea is to inject gas into wells which lifts the contaminated water in the well. During the process, VOCs are transferred from the water to the gas bubbles. The injected gas can be air or any specific gas such as nitrogen, carbon dioxide, or any other combination of gases. The injected gas can be at the ambient temperature or it can be warm or cold. The VOCs are then collected at the top of the well by vapor extraction.
An object of the present invention is to provide a simple, inexpensive and efficient technique for removing dissolved contaminants. That is, to use the gas-lift pumping and in-situ vapor stripping techniques for removing VOCs from ground water.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or will be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.